Since a liquid crystal panel is thin in thickness and light in weight, it is widely used as a display device such as a television receiver set, a computer display, a display of a personal digital assistant or the like. However, since a liquid crystal requires a given time after a drive voltage is applied to the liquid crystal until transmissivity of the liquid crystal reaches a predetermined level, the liquid crystal has a defect that it cannot display a fast-varying motion picture of high quality. In order to solve such a problem, when a gradation value varies between successive frames, a driving method, in which an overvoltage is applied to a liquid crystal so as to cause transmissivity of the liquid crystal to reach a predetermined level within a period of one frame, is employed (e.g., see Patent Document 1). More specifically, image data of a frame preceding by one frame is compared with image data of the current frame on a pixel-by-pixel basis. If it is decided that the gradation value varies between pixels, an amount of correction corresponding to an amount of change is added to the image data of the current frame. As a result, when a gradation value increases as compared with that of the pixel of a frame preceding by one frame, a drive voltage higher than a normal voltage is applied to the liquid crystal panel; whereas, when a gradation value decreases, a drive voltage lower than a normal voltage is applied to the liquid crystal panel.
The implementation of the aforementioned method requires a frame memory for outputting image data of the frame preceding by one frame. In recent years, since number of display pixels increases due to increased size of the liquid crystal panel, capacity of the frame memory is also required to be increased. Furthermore, since the increased number of display pixels increases the amount of data written in the frame memory or read out from the frame memory in a predetermined period (e.g., in a period of one frame), it is necessary to increase a data transmission rate by increasing a clock frequency for controlling the writing and reading operation. Such increased capacity of the frame memory and such increased data transmission rate lead to an increase in the cost of a liquid crystal display device.
In order to solve such problems, in an image processing circuit for driving a liquid crystal described in Patent Document 2, the necessary capacity of a frame memory is decreased by encoding image data and then storing the encoded image data in the frame memory. Further, the image data is corrected in accordance with a result of comparison between decoded image data of the current frame obtained by decoding the encoded image data and decoded image data of a frame preceding by one frame obtained by delaying the encoded image data by a period of one frame and then decoding it. As a result, when still picture data is inputted, an unwanted overvoltage caused by errors in encoding and decoding operation can be prevented from being applied to the liquid crystal.
Patent Document 1 is Japanese Patent Publication No. 2,616,652 (Paragraphs 0025-0026, FIG. 14), and
Patent Document 2 is Japanese Patent Application Kokai (Laid-Open) Publication No. 2004-163842 (Paragraphs 0021-0042, FIG. 1).